1. Field of the Invention
Embodiments of the invention are most generally related to the field of liquid crystals. More particularly, embodiments of the invention are directed to PCLC flake-based apparatus and methods and, even more particularly, to PCLC flake/host fluid suspensions enabling dual-frequency, reverse drive apparatus and methods, and to flake/fluid suspension encapsulation structures, methods, and applications.
2. Related Art
U.S. Pat. Nos. 6,665,042, 6,829,075, 7,042,617, and 7,238,316, all of which are commonly assigned to the assignee of the instant application, disclose and teach polymer liquid crystal (PLC)-based devices and methods, as well as manufacturing methods for flakes themselves. More particularly, the referenced patents disclose enabling details in regard to flake/fluid host suspensions, electronically addressable and switchable devices comprising the flake/fluid suspensions, flake/fluid suspension encapsulation formulations and techniques, doped flakes and methods for making and/or doping the flakes, as well as other related apparatus and methods. The subject matter of these patents are incorporated by reference herein in their entireties to the fullest extent allowed by applicable rules and laws.
Kosc et al., Polymer cholesteric liquid-crystal flake reorientation in an alternating-current electric field, JOURNAL OF APPLIED PHYSICS 98, 013509 (2005); Trajkovska-Petkoska et al., Enhanced Electro-Optic Behavior for Shaped Polymer Cholesteric Liquid-Crystal Flakes Made Using Soft Lithography, ADV. FUNCT. MATER., 15, No. 2, (2005); and, Cox et al., Modeling the effects of microencapsulation on the electro-optic behavior of polymer cholesteric liquid crystal flakes, JOURNAL OF APPLIED PHYSICS 106, 124911 (2009), further describe relevant background, teachings, and applications related to the embodied technology described herein. The subject matter of these publications are incorporated by reference herein in their entireties to the fullest extent allowed by applicable rules and laws.
Previous PCLC flake research has focused on manipulating flake motion with an AC or DC electric field. The flakes have been made to translate, reorient parallel with the electric field (to a non-reflective orientation from their original reflective orientation as suspended in a host medium), or display chaotic motion depending on the system conditions. Work has also been conducted on various host fluids having different dielectric properties as well as the effects of changing the dielectric properties of the PCLC flakes by doping. The different system combinations compatible with Maxwell-Wagner interfacial polarization reorientation have used various factors to adjust the speed of PCLC flake reorientation parallel with the electric field.
FIG. 1 shows an illustration of torques acting on a PCLC flake in an electro-optic test cell when the flake density is greater than that of fluid. As illustrated in FIG. 1a, when an electric field is applied, an electrostatic torque ΓE acts to orient the flake in line with the electric field E and the torques due to gravity, ΓG, and hydrodynamic drag, ΓH, resist. FIG. 1b illustrates that when the electric field is turned off, the torque due to gravity ΓG acts to re-orient the flake. Thus gravity must be relied upon to relax the flake back to its original, reflective orientation. Relaxation times of six to greater than 60 seconds have been observed, depending on the density differences between the flake and host fluid as well as flake dimensions, the viscosity of the host fluid, and other reported factors. Further, if the host medium/flake enclosure is not oriented perpendicular to gravity, the flakes may not relax to a full reflective position.
PCLC flake technology offers unique features and benefits for a number of applications including, but not limited to, switchable/tunable color filters, micropolarizers, and modulators in the fields of electro-optics and photonics; switchable paints, conformal coatings, and switchable smart windows for energy or privacy control in the coatings field; anti-counterfeiting, signature reduction, camouflage, encoded and encrypted information storage for military and security applications; and perhaps most predominantly, for reflective multi-color particle displays, electronic paper, flexible displays, and 3-D displays. Several attributes that make this technology attractive for information display devices include the highly saturated color obtainable through PCLC selective reflection at low flake concentration (3-5%) without polarizers or filters, response times on the order of hundreds of milliseconds, remarkably low drive voltage requirements (mV/μm), flake encapsulation capability, and others.
Therefore, it would be advantageous to provide an active, controllable reverse drive mechanism to relax or reorient the PCLC flakes back to their original reflective orientation after their active orientation to a non-reflective state in order for PCLC flake-based technology to reach its full potential in the field of reflective display.
Non-limiting, illustrative, and exemplary embodiments and aspects of the invention are disclosed below.